Hyssop Side Effects, Interactions and Warnings

Commercial preparations of essences of hyssop has caused human intoxication in eight cases, from which tonico-clonic convulsions were the major symptom. The experimental study of the toxic properties of commercialized essential oils of sage and hyssop has revealed that their convulsant action was of central nervous system origin from pinocamphone in hyssop oil.- Clin Toxicol 1981 Dec;18(12):1485-98 -- Toxicity of some essential plant oils. Clinical and experimental study.

The oil has been reported to cause seizures in laboratory animals as well as in humans. Do not use if you have epilepsy or are pregnant or nursing.
- Tisserand R, Balacs T. -- Essential Oil Safety: A Guide for Health Care Professionals. -- Churchill Livingstone.